Prosecutors looking into sweeping allegations of financial fraud involving former President Lee Myung-bak are set to complete their investigation as early as this week and likely to summon Lee for questioning soon.

Former President Lee Myung-bak. Yonhap

If Lee is summoned by the prosecution, he would be the fourth president of South Korea to be questioned by investigators following detained ex-leader Park Geun-hye.

Although no charges have been filed yet, Lee faces allegations that he ran a 12 billion won ($11 million) slush fund hidden in auto parts company DAS, whose real owner is suspected to be Lee, while on paper it belongs to his elder brother, according to the Seoul Central District Prosecutors’ Office.

Lee is also suspected of taking bribes that were part of the National Intelligence Service’s funds designated for the spy agency’s espionage activities and not audited by the National Assembly.

Former NIS officials testified to prosecutors that the agency’s funds were delivered to Lee’s presidential aides during his tenure between 2008 and 2013 under orders from the top presidential office.

Separately, prosecutors raided last week the home of Lee’s son-in-law, Lee Sang-joo, a senior executive at Samsung Electronics. The son-in-law took part of an alleged kickback, 1.45 billion won, between 2008 and 2011 from a former head of Woori Finance Holdings. Lee Pal-seung was appointed as the head of the then state-controlled Woori under the Lee administration after allegedly bribing the ex-president through his presidential aides.

Others linked to the corruption allegations include Lee’s elder brother and his son -- Lee Sang-deuk, the largest shareholder of DAS, and Lee Si-hyung, an executive of the company -- over a slush fund and unfair trade related to the auto-parts maker.

It is nothing new for South Koreans to see their presidents grilled and sent to jail over corruption charges. Most recently, prosecutors demanded the court give 30 years in prison to ex-President Park, Lee’s successor, for “disrupting constitutional order and damaging the public’s trust in state power” as part of an ongoing corruption trial.

Park was expelled from power on March 10, arrested the next day and indicted on April 17 on a total of 18 charges, prompting an early presidential election last year.

Before Park, two other state leaders, Chun Doo-hwan and his successor Roh Tae-woo, were charged after their tenures between 1995 and 1996 for taking bribes from local conglomerates. They were later pardoned in 1997.